1. Field of the Invention
The present invention relates to a semiconductor laser element which is suitable for use in optical communications.
2. Description of the Related Art
In recent years, extension of optical fibers to individual homes has been becoming more prevalent, and the individual homes have been more intellectually empowered through optical networks. Although the transmission rates of the optical-fiber communications are currently about 100 Mbps, demands for transmission of greater amount of information at higher transmission rates will increase in the future.
In the above circumstances, it is important to suppress the cost of placement of optical fibers. Therefore, attention is currently being focused on the plastic fibers rather than the quartz-based fibers, which are currently mainstream, since the plastic fibers are less expensive. In addition, since the plastic fibers have large core diameters and are easy to couple to semiconductor lasers, it is possible to suppress the construction cost.
However, the plastic fibers cause great internal loss. For example, Japanese Unexamined Patent Publication No. 6(1994)-275907 discloses a 660-nm band semiconductor laser in which an active layer has a compressed-strain quantum-well structure made of InGaP, and which is currently used as a light source for DVD recording. When this semiconductor laser is used as a light source for optical communication, internal loss of about 200 dB/km occurs in plastic fibers, and data can be transmitted at most to a distance of about 30 m.
The transmission distance can be increased by decreasing the wavelength of the transmitted light. For example, if the wavelength of the transmitted light can be decreased to 650 nm, it is possible to decrease the internal loss in the optical fiber to 150 dB/km, i.e., about ¾ of the internal loss in the case where the wavelength of the transmitted light is 660 nm. However, when the oscillation wavelengths of semiconductor lasers are decreased to 645 to 650 nm, the characteristics of the semiconductor lasers deteriorate due to various constraints. Therefore, the semiconductor lasers having an oscillation wavelength of 645 to 650 nm are not yet practicable.
For example, it is possible to consider decrease in the oscillation wavelength of the semiconductor laser disclosed in Japanese Unexamined Patent Publication No. 6(1994)-275907 to 645 nm by reducing the thickness of the quantum-well active layer. However, since it is impossible to obtain sufficient gain when the thickness of the quantum-well active layer is small, it is necessary to compensate for the insufficiency of the gain by increasing the number of quantum wells. The increase in the number of quantum wells increases the driving current. Therefore, it is impossible to secure sufficient reliability. In addition, in order to realize the oscillation wavelength of 645 nm, it is necessary to reduce the thickness of the quantum-well active layer to about 2 nm. Nevertheless, since, in such a case, the thickness of the quantum-well active layer is near to the atomic sizes, it is difficult to control the thickness, and resultantly the oscillation wavelengths of individual semiconductor laser elements can greatly vary.
Alternatively, the oscillation wavelengths of semiconductor lasers can be decreased by replacing the quantum-well active layer with an unstrained InGaP layer. However, in this case, it is impossible to obtain a satisfactory current-optical output characteristic since the effect of reducing the threshold value in the strained quantum-well structure is lost.